This circuit is for traditional old-style telephone lines found in an American 1980s documentation. Components are common and the optical coupler can be replaced with an equivalent.
This circuit was found in a 1965’s General Electric documentation. It is intended for low voltage...
This protective circuit for power lines was found in a Radio Electronics from November 1957. The...
Found in the March 1974 Radio Electronics magazine, this circuit is designed to measure the level...